The present invention relates to passive heat removal from a nuclear reactor containment airspace with use of an isolation condenser that cools a mixture of steam and heated gasses present in the containment airspace as a consequence of a reactor system loss-of-coolant accident (LOCA), the steam and heated gas mixture entering the isolation condenser from the containment airspace through an open entry conduit and communicating within the isolation condenser with the primary side of a heat exchanger section, whereupon within the heat exchange section heat is transferred to a cooling water medium supplied to the secondary side of the heat exchange section and steam is condensed and returned as cool condensate back to that containment airspace through a return conduit and non-condensable gasses are discharged downstream from the heat exchange section through a conduit communicating to an enclosed space within the containment maintained at a lower pressure than the containment airspace pressure.
The invention is particularly directed to effecting more immediate and/or advantageous utilization, following a LOCA event, of the return condensate for cooling of the reactor and/or containment airspace as well as use in a water trap placed in the isolation condenser condensate return conduit with the water trap so proportioned as to restrict backflow entry into the condensate return conduit of steam and heated gasses present in the containment airspace, where lacking such water trap effect, the containment gas mixture could bypass the condenser heat exchange section while gaining entry to the enclosed space that is being maintained at lower pressure without first being cooled within the heat exchange section.
Commonly assigned applications Ser. Nos. 07/325,729 and 07/432,246 filed Mar. 20, 1989 and Nov. 6, 1989, respectively, disclose nuclear systems wherein various manners and means can be employed for passive heat removal from a system containment space in the happening of a loss-of-coolant accident or event such as a break in a steam pipe or the pressure vessel, or the loss of coolant in the pressure vessel by other cause. Both of the disclosed systems dissipate initial heat by venting steam from the pressure vessel to a suppression pool. Also, both systems employ an elevated gravity driven cooling pool which when pressure in the pressure vessel reduces to a certain level, flows by force of gravity into the pressure vessel to replace any loss of coolant therein. Both systems also employ elevated isolation condensers which are submerged in a large water supply, for initial cooling and for decay heat dissipation as well. In the '729 application system, the pressure vessel is vented through a designated line to the isolation condenser and steam is cooled and returned to the pressure vessel. After a time when pressure in the pressure vessel is below that in the containment space, an opened vent valve on the pressure vessel admits steam and heated gasses present in the containment to and through the pressure vessel from whence it flows to the isolation condenser, is condensed and returned as cooled condensate to the pressure vessel. The '246 application system on the other hand, has no communication with the pressure vessel. An open entry conduit is disposed in the containment space and from outset of an event, steam and heated gasses such as nitrogen present in the containment, can flow into this open entry conduit and access the isolation condenser for cooling, with the cooled condensate then being passed through a return conduit into an enclosed volume or space within the containment commonly referred to as a wetwell, within which is a pool of water, i.e., a suppression pool, over which suppression pool there is a gas volume or wetwell airspace. Non-condensable gases such as nitrogen are separated from the condensate and vented to the suppression pool through the force of a pressure differential that is at certain times maintained or exists between the respective containment airspace and the wetwell airspace.
The systems of both applications work well for the purposes for which designed, but it is noted that removal of containment space heat by way of the isolation condenser, does not occur for some time following an event in the '729 application system and until such time as pressure vessel pressure is lower than containment space pressure so that steam and heated gasses can pass therefrom into the pressure vessel for passage to the isolation condenser and cooling therein. The '246 application system having an open entry conduit in the containment connected to the isolation condenser, effects containment space cooling at outset of an event since containment space steam and heated gasses always have free access to this isolation condenser entry flow path. The '246 application system returns cooled condensate to the suppression pool, i.e., a location from which it may be required, during system cooldown, that the water therein be flowed into the pressure vessel to protectively cover the reactor core. Depending on the pressure in the pressure vessel and the location of the core in the pressure vessel relative to suppression pool level, the suppression pool water may lack sufficient hydrostatic head to enter the pressure vessel and cover the core, the condensate return from the isolation condenser having expended substantially all the hydrostatic head it possessed at isolation condenser elevation on flow entry to and commingling with the suppression pool water its addition to the pool not increasing pool head of any significance to the suppression pool. The condensate return from the isolation condenser in the '729 application system possesses vessel pressure countering inflow head since it is direct returned to the pressure vessel without commingling with any intermediate stock of coolant. It is seen then that both application systems have advantage. The '729 system allows for early replenishment of pressure vessel coolant to cover the reactor core, but there is some time lapse before the isolation condenser can function to dissipate containment space steam and heated gasses thermal loads. The '246 system is capable of immediate extraction of containment space heat present as steam and heated gasses, but the cool condensate generated by this cooling cannot most effectively be used for core coolant purpose by reason of its being returned to the suppression pool.
It is desirable therefore, that improved loss-of-coolant event passive cooling capacity and flexibility involving use of isolation condensers be provided in a nuclear reactor system.